Project Owlnet is in the business of banding saw-whet owls to learn more about them. Kayla Randall of Smithsonian Magazine explained how they accomplish this:

The group uses an audio lure to entice the birds, capturing them in mist nets to bring back to the banding station. Once there, they place aluminum bands on the birds—’friendship bracelets for science,’ as they’re called within the project. Project participants also measure the owls’ bills, wings and tails.

While I did not know how one would catch and band a saw-whet owl despite having written All About the Saw-whet Owl back in 2021, that passage is not the thing I learned of the day. What caught my attention was how the researchers estimate how old individual Saw-whet owls are:

They use a blacklight to look at the underside of the owls’ wings and see their molt pattern, which helps determine their ages, a difficult task. Old feathers don’t glow as brightly under the light because the pigment has faded, while new feathers have a brighter glow, Boyle Acuti says.

I would not have guessed that they use blacklights to estimate the age of Saw-whet owls. [Source: Kaya Randall for Smithsonian Magazine]